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Tue, 31 Aug 2021 05:28:50 -0700 (PDT) Received: from phenom.ffwll.local ([2a02:168:57f4:0:efd0:b9e5:5ae6:c2fa]) by smtp.gmail.com with ESMTPSA id y11sm22017864wru.0.2021.08.31.05.28.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 31 Aug 2021 05:28:50 -0700 (PDT) Date: Tue, 31 Aug 2021 14:28:47 +0200 From: Daniel Vetter To: Desmond Cheong Zhi Xi Cc: maarten.lankhorst@linux.intel.com, mripard@kernel.org, tzimmermann@suse.de, airlied@linux.ie, sumit.semwal@linaro.org, christian.koenig@amd.com, jani.nikula@linux.intel.com, joonas.lahtinen@linux.intel.com, rodrigo.vivi@intel.com, chris@chris-wilson.co.uk, ville.syrjala@linux.intel.com, matthew.auld@intel.com, dan.carpenter@oracle.com, tvrtko.ursulin@intel.com, matthew.d.roper@intel.com, lucas.demarchi@intel.com, karthik.b.s@intel.com, jose.souza@intel.com, manasi.d.navare@intel.com, airlied@redhat.com, aditya.swarup@intel.com, andrescj@chromium.org, linux-graphics-maintainer@vmware.com, zackr@vmware.com, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, intel-gfx@lists.freedesktop.org, linux-media@vger.kernel.org, linaro-mm-sig@lists.linaro.org, skhan@linuxfoundation.org, gregkh@linuxfoundation.org, linux-kernel-mentees@lists.linuxfoundation.org, Daniel Vetter Subject: Re: [PATCH v8 7/7] drm: remove drm_file.master_lookup_lock Message-ID: Mail-Followup-To: Desmond Cheong Zhi Xi , maarten.lankhorst@linux.intel.com, mripard@kernel.org, tzimmermann@suse.de, airlied@linux.ie, sumit.semwal@linaro.org, christian.koenig@amd.com, jani.nikula@linux.intel.com, joonas.lahtinen@linux.intel.com, rodrigo.vivi@intel.com, chris@chris-wilson.co.uk, ville.syrjala@linux.intel.com, matthew.auld@intel.com, dan.carpenter@oracle.com, tvrtko.ursulin@intel.com, matthew.d.roper@intel.com, lucas.demarchi@intel.com, karthik.b.s@intel.com, jose.souza@intel.com, manasi.d.navare@intel.com, airlied@redhat.com, aditya.swarup@intel.com, andrescj@chromium.org, linux-graphics-maintainer@vmware.com, zackr@vmware.com, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, intel-gfx@lists.freedesktop.org, linux-media@vger.kernel.org, linaro-mm-sig@lists.linaro.org, skhan@linuxfoundation.org, gregkh@linuxfoundation.org, linux-kernel-mentees@lists.linuxfoundation.org References: <20210826020122.1488002-1-desmondcheongzx@gmail.com> <20210826020122.1488002-8-desmondcheongzx@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Operating-System: Linux phenom 5.10.0-8-amd64 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 31, 2021 at 02:02:39PM +0800, Desmond Cheong Zhi Xi wrote: > On 26/8/21 9:21 pm, Daniel Vetter wrote: > > On Thu, Aug 26, 2021 at 10:01:22AM +0800, Desmond Cheong Zhi Xi wrote: > > > Previously, master_lookup_lock was introduced in > > > commit 0b0860a3cf5e ("drm: serialize drm_file.master with a new > > > spinlock") to serialize accesses to drm_file.master. This then allowed > > > us to write drm_file_get_master in commit 56f0729a510f ("drm: protect > > > drm_master pointers in drm_lease.c"). > > > > > > The rationale behind introducing a new spinlock at the time was that > > > the other lock that could have been used (drm_device.master_mutex) was > > > the outermost lock, so embedding calls to drm_file_get_master and > > > drm_is_current_master in various functions easily caused us to invert > > > the lock hierarchy. > > > > > > Following the conversion of master_mutex into a rwsem, and its use to > > > plug races with modesetting rights, we've untangled some lock > > > hierarchies and removed the need for using drm_file_get_master and the > > > unlocked version of drm_is_current_master in multiple places. > > > > > > Hence, we can take this opportunity to clean up the locking design by > > > replacing master_lookup_lock with drm_device.master_rwsem. > > > > > > Signed-off-by: Desmond Cheong Zhi Xi > > > --- > > > drivers/gpu/drm/drm_auth.c | 19 +++++++------------ > > > drivers/gpu/drm/drm_file.c | 1 - > > > drivers/gpu/drm/drm_internal.h | 1 + > > > drivers/gpu/drm/drm_ioctl.c | 4 ++-- > > > drivers/gpu/drm/drm_lease.c | 18 ++++++++---------- > > > include/drm/drm_file.h | 9 +-------- > > > 6 files changed, 19 insertions(+), 33 deletions(-) > > > > > > diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c > > > index f2b2f197052a..232416119407 100644 > > > --- a/drivers/gpu/drm/drm_auth.c > > > +++ b/drivers/gpu/drm/drm_auth.c > > > @@ -61,10 +61,9 @@ > > > * trusted clients. > > > */ > > > -static bool drm_is_current_master_locked(struct drm_file *fpriv) > > > +bool drm_is_current_master_locked(struct drm_file *fpriv) > > > { > > > - lockdep_assert_once(lockdep_is_held(&fpriv->master_lookup_lock) || > > > - lockdep_is_held(&fpriv->minor->dev->master_rwsem)); > > > + lockdep_assert_held_once(&fpriv->minor->dev->master_rwsem); > > > return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master; > > > } > > > @@ -83,9 +82,9 @@ bool drm_is_current_master(struct drm_file *fpriv) > > > { > > > bool ret; > > > - spin_lock(&fpriv->master_lookup_lock); > > > + down_read(&fpriv->minor->dev->master_rwsem); > > > > Looking at the 3 patches and the need to have a locked version of pretty > > much everything I'm wondering: Can't we just drop the spinlock completely, > > and everywhere we've taking it thus far replace it with a > > lockdep_assert_held_once? > > > > The thing is, if there's any path left that doesn't hold the rwsem in at > > least read mode we have a bug. And the right way to fix such a bug is to > > grab the rwsem sufficiently high up in the callchain. That way I think we > > should be able to avoid all these tedious changes to everything, including > > touching i915 and vmwgfx drivers. > > > > Or am I missing something big time? > > -Daniel > > > > Thanks for taking a look at all the patches and for the suggestions, Daniel. > > Just my two cents. I think it makes sense to replace the lock with the > lockdep assertion. This avoids the weirdness with the lock being taken both > as an outer lock and sometimes as a deeply embedded inner lock. > > But we'll probably have to fix some stuff because I don't think we always > hold the rwsem in the places where the spinlock is grabbed (i.e. when > drm_is_current_master or drm_file_get_master is called). Yeah right I forgot about that again when coming up with this idea. All the ioctl that read kms state need a read lock too. Maybe those ioctl could just grab the master rwsem themselves? This should work now that we've untangled the drm_global_mutex situation I think. > I'll split the series as suggested so we can test things up to PATCH 4 > ("drm: avoid races with modesetting rights"). For the rest of the series to > remove the spinlock, I'll take a closer look and probably send out a patch > later this week. Sounds great! -Daniel > > Best wishes, > Desmond > > > > ret = drm_is_current_master_locked(fpriv); > > > - spin_unlock(&fpriv->master_lookup_lock); > > > + up_read(&fpriv->minor->dev->master_rwsem); > > > return ret; > > > } > > > @@ -120,7 +119,7 @@ int drm_authmagic(struct drm_device *dev, void *data, > > > DRM_DEBUG("%u\n", auth->magic); > > > down_write(&dev->master_rwsem); > > > - if (unlikely(!drm_is_current_master(file_priv))) { > > > + if (unlikely(!drm_is_current_master_locked(file_priv))) { > > > up_write(&dev->master_rwsem); > > > return -EACCES; > > > } > > > @@ -178,9 +177,7 @@ static int drm_new_set_master(struct drm_device *dev, struct drm_file *fpriv) > > > new_master = drm_master_create(dev); > > > if (!new_master) > > > return -ENOMEM; > > > - spin_lock(&fpriv->master_lookup_lock); > > > fpriv->master = new_master; > > > - spin_unlock(&fpriv->master_lookup_lock); > > > fpriv->is_master = 1; > > > fpriv->authenticated = 1; > > > @@ -343,9 +340,7 @@ int drm_master_open(struct drm_file *file_priv) > > > if (!dev->master) { > > > ret = drm_new_set_master(dev, file_priv); > > > } else { > > > - spin_lock(&file_priv->master_lookup_lock); > > > file_priv->master = drm_master_get(dev->master); > > > - spin_unlock(&file_priv->master_lookup_lock); > > > } > > > up_write(&dev->master_rwsem); > > > @@ -413,13 +408,13 @@ struct drm_master *drm_file_get_master(struct drm_file *file_priv) > > > if (!file_priv) > > > return NULL; > > > - spin_lock(&file_priv->master_lookup_lock); > > > + down_read(&file_priv->minor->dev->master_rwsem); > > > if (!file_priv->master) > > > goto unlock; > > > master = drm_master_get(file_priv->master); > > > unlock: > > > - spin_unlock(&file_priv->master_lookup_lock); > > > + up_read(&file_priv->minor->dev->master_rwsem); > > > return master; > > > } > > > EXPORT_SYMBOL(drm_file_get_master); > > > diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c > > > index 90b62f360da1..8c846e0179d7 100644 > > > --- a/drivers/gpu/drm/drm_file.c > > > +++ b/drivers/gpu/drm/drm_file.c > > > @@ -176,7 +176,6 @@ struct drm_file *drm_file_alloc(struct drm_minor *minor) > > > init_waitqueue_head(&file->event_wait); > > > file->event_space = 4096; /* set aside 4k for event buffer */ > > > - spin_lock_init(&file->master_lookup_lock); > > > mutex_init(&file->event_read_lock); > > > if (drm_core_check_feature(dev, DRIVER_GEM)) > > > diff --git a/drivers/gpu/drm/drm_internal.h b/drivers/gpu/drm/drm_internal.h > > > index 17f3548c8ed2..5d421f749a17 100644 > > > --- a/drivers/gpu/drm/drm_internal.h > > > +++ b/drivers/gpu/drm/drm_internal.h > > > @@ -132,6 +132,7 @@ int drm_crtc_queue_sequence_ioctl(struct drm_device *dev, void *data, > > > struct drm_file *filp); > > > /* drm_auth.c */ > > > +bool drm_is_current_master_locked(struct drm_file *fpriv); > > > int drm_getmagic(struct drm_device *dev, void *data, > > > struct drm_file *file_priv); > > > int drm_authmagic(struct drm_device *dev, void *data, > > > diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c > > > index 8bea39ffc5c0..c728437466c3 100644 > > > --- a/drivers/gpu/drm/drm_ioctl.c > > > +++ b/drivers/gpu/drm/drm_ioctl.c > > > @@ -386,7 +386,7 @@ static int drm_setversion(struct drm_device *dev, void *data, struct drm_file *f > > > int if_version, retcode = 0; > > > down_write(&dev->master_rwsem); > > > - if (unlikely(!drm_is_current_master(file_priv))) { > > > + if (unlikely(!drm_is_current_master_locked(file_priv))) { > > > retcode = -EACCES; > > > goto unlock; > > > } > > > @@ -540,7 +540,7 @@ static int drm_ioctl_permit(u32 flags, struct drm_file *file_priv) > > > /* MASTER is only for master or control clients */ > > > if (unlikely((flags & DRM_MASTER) && > > > - !drm_is_current_master(file_priv))) > > > + !drm_is_current_master_locked(file_priv))) > > > return -EACCES; > > > /* Render clients must be explicitly allowed */ > > > diff --git a/drivers/gpu/drm/drm_lease.c b/drivers/gpu/drm/drm_lease.c > > > index 15bf3a3c76d1..0eecf320b1ab 100644 > > > --- a/drivers/gpu/drm/drm_lease.c > > > +++ b/drivers/gpu/drm/drm_lease.c > > > @@ -498,12 +498,12 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev, > > > return PTR_ERR(lessee_file); > > > down_read(&dev->master_rwsem); > > > - if (unlikely(!drm_is_current_master(lessor_priv))) { > > > + if (unlikely(!drm_is_current_master_locked(lessor_priv))) { > > > ret = -EACCES; > > > goto out_file; > > > } > > > - lessor = drm_file_get_master(lessor_priv); > > > + lessor = lessor_priv->master; > > > /* Do not allow sub-leases */ > > > if (lessor->lessor) { > > > DRM_DEBUG_LEASE("recursive leasing not allowed\n"); > > > @@ -565,7 +565,6 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev, > > > /* Hook up the fd */ > > > fd_install(fd, lessee_file); > > > - drm_master_put(&lessor); > > > up_read(&dev->master_rwsem); > > > DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl succeeded\n"); > > > return 0; > > > @@ -600,7 +599,8 @@ int drm_mode_list_lessees_ioctl(struct drm_device *dev, > > > if (!drm_core_check_feature(dev, DRIVER_MODESET)) > > > return -EOPNOTSUPP; > > > - lessor = drm_file_get_master(lessor_priv); > > > + lockdep_assert_held_once(&dev->master_rwsem); > > > + lessor = lessor_priv->master; > > > DRM_DEBUG_LEASE("List lessees for %d\n", lessor->lessee_id); > > > mutex_lock(&dev->mode_config.idr_mutex); > > > @@ -624,7 +624,6 @@ int drm_mode_list_lessees_ioctl(struct drm_device *dev, > > > arg->count_lessees = count; > > > mutex_unlock(&dev->mode_config.idr_mutex); > > > - drm_master_put(&lessor); > > > return ret; > > > } > > > @@ -650,7 +649,8 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev, > > > if (!drm_core_check_feature(dev, DRIVER_MODESET)) > > > return -EOPNOTSUPP; > > > - lessee = drm_file_get_master(lessee_priv); > > > + lockdep_assert_held_once(&dev->master_rwsem); > > > + lessee = lessee_priv->master; > > > DRM_DEBUG_LEASE("get lease for %d\n", lessee->lessee_id); > > > mutex_lock(&dev->mode_config.idr_mutex); > > > @@ -678,7 +678,6 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev, > > > arg->count_objects = count; > > > mutex_unlock(&dev->mode_config.idr_mutex); > > > - drm_master_put(&lessee); > > > return ret; > > > } > > > @@ -703,11 +702,11 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev, > > > return -EOPNOTSUPP; > > > down_write(&dev->master_rwsem); > > > - if (unlikely(!drm_is_current_master(lessor_priv))) { > > > + if (unlikely(!drm_is_current_master_locked(lessor_priv))) { > > > ret = -EACCES; > > > goto unlock; > > > } > > > - lessor = drm_file_get_master(lessor_priv); > > > + lessor = lessor_priv->master; > > > mutex_lock(&dev->mode_config.idr_mutex); > > > lessee = _drm_find_lessee(lessor, arg->lessee_id); > > > @@ -728,7 +727,6 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev, > > > fail: > > > mutex_unlock(&dev->mode_config.idr_mutex); > > > - drm_master_put(&lessor); > > > unlock: > > > up_write(&dev->master_rwsem); > > > diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h > > > index d12bb2ba7814..e2d49fe3e32d 100644 > > > --- a/include/drm/drm_file.h > > > +++ b/include/drm/drm_file.h > > > @@ -227,16 +227,12 @@ struct drm_file { > > > * @master: > > > * > > > * Master this node is currently associated with. Protected by struct > > > - * &drm_device.master_rwsem, and serialized by @master_lookup_lock. > > > + * &drm_device.master_rwsem. > > > * > > > * Only relevant if drm_is_primary_client() returns true. Note that > > > * this only matches &drm_device.master if the master is the currently > > > * active one. > > > * > > > - * To update @master, both &drm_device.master_rwsem and > > > - * @master_lookup_lock need to be held, therefore holding either of > > > - * them is safe and enough for the read side. > > > - * > > > * When dereferencing this pointer, either hold struct > > > * &drm_device.master_rwsem for the duration of the pointer's use, or > > > * use drm_file_get_master() if struct &drm_device.master_rwsem is not > > > @@ -248,9 +244,6 @@ struct drm_file { > > > */ > > > struct drm_master *master; > > > - /** @master_lock: Serializes @master. */ > > > - spinlock_t master_lookup_lock; > > > - > > > /** @pid: Process that opened this file. */ > > > struct pid *pid; > > > -- > > > 2.25.1 > > > > > > -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch